We've been married 32 years next month. My favorite anniversaries were the two that he forgot:
#1. We celebrated our 4th anniversary a few days early because DH was in school at the time and our anniversary fell during one of his classes. I remembered at 10:00 that night as we were praying before bed. I could hardly wait for him to say amen so I could wish him "Happy Anniversary!" He was dumbfounded.
#2. I remembered, but he did not. I decided to make meatloaf for dinner since I got the impression that we were not going out. He loves meatloaf, but I hate it, so I only make it for his birthday or Father's Day. So he sits down to a nice meatloaf dinner, with at least three or four of our kids also with us, and is so delighted. "Meatloaf! What's the special occasion?"
I wished him Happy Anniversary, and he was absolutely floored. And then our younger son piped in, "I was going to say something, but you never forget, so I didn't." DH still hasn't forgiven him.